I have not lived in or visited Turkey myself in a several years so others probably will know better. But my understanding from family and friends and based on previous experiences is that the duty free shop will be priced in EUR. There can be a lot of difference in the Turkish vs USA prices based on item, so I suggest you send him a list of what you are looking for and what the USD price is so he can do the up-to-date conversion to EUR or, less likely TRY. If he will actually be in Istanbul, not passing though the airport only, I suggest he go to one of the Dior boutiques. There are at least two iirc and they have a lot of advantages. First, they afaik are priced in TRY, and you can get a bigger discount that way. They also used to update prices less frequently than duty free, so you have a better chance of taking advantage of the currency fluctuations and weak lira. He can do detax at the airport, it is not too much of a hassle. The other advantage of the stores is that most (all?) of them are actually owned by the Turkish luxury department store Beymen, which runs periodic sales. Before I moved to the US I took advantage of their end of season sales but also "butterfly days" (kelebek gunleri), which were days once a month when things were put on sale. Finally, the boutiques in town will be more advantageous if you are looking for shoes and, like me, have big feet for a Turkish lady anyway. Duty free will likely be sold out in the sizes above 39, but you can find almost any shoe in the bigger sizes in the boutiques. Turkish luxury shopping is kind of a crapshoot, all dependent on how the currency has moved since the stores last updated their prices (I mean the actual boutiques in Turkey according to the exchange rate, not the house-wide price increase.)
I hope he enjoys his trip!
